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CRUDE LIGHT TAR OILS

319

(1)  Up to 170°-—light oils.

(2)   From 170° to 230°—middle or carbolic oils.

(3)  From 230 to 270°—heavy or creosote oils.

(4)   Above 270° (distilled without thermometer)—green or anthracene
oils.

The distillation is discontinued when, almost all the tar being distilled,
the drops passing over become intensely red. The residue in the retort,
representing the pitch, is weighed.

Industrially the distillation test is carried out on larger quantities (0-5 to
5 kilos) in a metallic vessel, so that results in greater accord with those of the
works may be obtained.1

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* *

The specific gravity of coal tar (dry) usually varies from i-ioo to 1-280, but
in exceptional cases may be below i. The composition of the tar varies according
to the character of the coal yielding it and to the mode of heating (whether in
vertical or horizontal retorts), and similar variation is shown by the yield of
distillation products. Tar contains 10-35% of free carbon and the quantity
of water permissible in it when sold to the distilleries is 4-5%.

CRUDE  LIGHT  TAR  OILS

Analysis of these products includes the following determinations :

1.  Determination of the Specific Gravity.—By means of a hydro-
meter or Westphal balance at 15° C.                            • • i

2.  Distillation.—100   c.c.   are   fractionally   distilled   from   a   glass
or,  better,   copper  vessel  of  150  c.c.  capacity, furnished with a ther-
mometer and connected with a condenser.   The portion passing over up
to 120° is the crude benzole (with toluole, etc.) and that between 120° and
170°, the naphtha (solvent naphtha) ;  the residue is regarded as middle oils.
The distillates may be tested by the reactions for detecting the presence
of any light petroleum oils (benzine) or oil of turpentine (see later : Benzole,
3, c, and also Oil of Turpentine, Vol. II).

3.  Determination   of  the  Phenols.—The  fractions  obtained  from
the preceding distillation are reunited, the containing vessels being rinsed
out with xylene and the whole introduced into a 500 c.c. graduated cylinder
with a ground stopper and repeatedly shaken with 100 c.c. of caustic soda
solution  (D 1-2).    After being left at rest for some time, the volume of
soda solution underneath is read, the increase in its volume giving the
percentage of phenols by volume.

For a more exact determination the alkaline layer is collected and
evaporated on a water-bath until addition of water no longer produces
turbidity. When cold, the liquid is acidified with hydrochloric acid and
treated with sodium chloride, the layer of phenols which separates being
measured.

4.  Determination of the Bases.—The oil freed from phenols by the
treatment just described is repeatedly shaken with 30 c.c. of 20% sulphuric

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1 Lunge :   Technical Methods of Chemical Analysis (London, 1911),  Vol. II, p.air by means of a capillary tube dipping